Conventional broadband and wireless communication receivers use a high performance front end amplifier, often called a low noise amplifier (LNA). Such amplifiers should have excellent noise performance and linearity performance across both a wide frequency bandwidth and a wide range of input signal power.
Building a front end variable gain amplifier (VGA) that can simultaneously maintain good noise, good linearity, and wide bandwidth is a very challenging task. When the input signal power can be either very small or very large, accomplishing this task with a fixed-gain amplifier becomes next to impossible. Large gains applied to large signals tend to lead to high distortion levels. Small gains and small signal levels tend to lead to poor signal-to-noise ratios (SNRs).
A front end amplifier can manage the noise-linearity tradeoff by varying its gain depending on input signal strength. If an input signal is weak, a LNA needs higher gain and low noise performance. If an input signal is strong, a LNA needs a reduced gain correspondingly to deliver an optimum signal amplitude to the next stage while maintaining good linearity performance to handle a relatively large signal. If the gain must be changed across an order of magnitude or more, it is difficult to implement a linear gain control function in a broadband LNA while maintaining both noise and a linearity performance requirements.
Referring to FIGS. 1 and 2, a circuit 10 is shown illustrating a conventional amplifier design. The circuit 10 generates a signal OUT in response to a signal IN. The gain of the amplifier 10 is varied by changing the shunt feedback resistor RF. Such conventional current feedback amplifiers are a common choice for a broadband LNA design because of the wide band performance. However, for a large amount of gain change, it becomes increasingly difficult to maintain good linearity, noise, and bandwidth by changing the gain by only varying the value of a feedback resistor RF.
In particular, if the gain of the amplifier 10 is varied only with the feedback resistor RF (i.e., Gain=RF/RIN), then for higher gain settings a large value (i.e., 3KΩ) of the resistor RF is needed. When the signal IN is amplified by using a large resistor RF, as the gain becomes bigger headroom limitations can cause the signal OUT to become more nonlinear. Furthermore, high values of the feedback resistor RF almost always lead to bandwidth shrinkage (i.e., a smaller frequency range where the desired gain is maintained). In the case of the low gain setting, as the value of the resistor RF is set to a low value (i.e., 100Ω), the noise contribution from the resistor RF is increased which can unacceptably deteriorate SNR at the low gain setting. Furthermore, maintaining stability as the feedback resistor RF is changed by an order of magnitude or more becomes very difficult, and would likely result in small bandwidth.
As a result, a current feedback amplifier that adjusts the gain with only the resistor RF may not meet the strict requirements demanded by modern high-speed variable-gain LNAs.
It would be desirable to implement a variable gain current feedback amplifier that maintains a wide bandwidth and stability.